Welding helmets have been used in the past to protect the eyes and face of a person doing welding (hereinafter referred to as a welder) from the very bright light occurring during welding, e.g., emanating from the welding are, and from possible particles that may be flung toward the welder during welding. Early welding helmets had a lens through which a welder would view the work being welded and a protective shield material, such as metal, plastic or other solid material, that contained the lens and protected the welder's face from the light emitted by the welding operation and from particles. Typically the lens was a material that would transmit a relatively small amount of the incident light and, thus, when the welding was occurring would permit enough light to pass to the welder's eyes to observe the welding operation while blocking a substantial amount of the light occurring during welding so that the eyes would not be injured. Sometimes the lens was used in spectacle frames for eye protection without the face protecting shield.
Early welding helmets suffered from the disadvantage that the lenses were of fixed light transmission characteristic, i.e., darkness. Since the lenses were adequately dark (non-transmissive of or able to block transmission of some light) to perform eye protection function, it was difficult or even impossible in the absence of the welding arc to see through the lens such as to start a welding torch, arc, etc.
Efforts were made to provide variable light transmission characteristics in welding lenses and/or in welding helmets using such lenses. Several examples included variable mechanical devices, such as mechanical shutters located over the viewing area of the welding helmet to control the aperture through which light may be transmitted to the eyes of the welder. Sensors detected the occurrence of the welding light and caused a circuit automatically to close the shutter aperture.
Another early mechanical shutter used relatively rotatable polarizers in optical series. Depending on the brightness or "clearness" desired (or darkness or light attenuation desired) one polarizer was rotated relative to the other. A sensor for detecting incident light and a circuit responsive to the sensor controlled the relative rotation of the polarizers.
An example of a variable solid crystal welding lens with polarizers in protective eye glasses (spectacles) or goggles is disclosed in Marks et at. U.S. Pat. No. 3,245,315.
Other efforts made to provide variable light transmission characteristics for a welding lens have used variable liquid crystal light shutter devices. Two examples of twisted nematic liquid crystal cells used in welding lens assemblies in welding helmets are disclosed in U.S. Pat. Nos. Re. 29,684 (Gordon) and 4,039,254 (Harsch). In the Gordon light shutter a twisted nematic liquid crystal cell, sandwiched between crossed polarizers, rotates the plane of polarized light received from one polarizer to pass it through the second polarizer, thus allowing a welder to see in the absence of a welding arc. In response to welding light being detected by a sensor, a circuit energizes the twisted nematic liquid crystal cell so that polarized light remains unrotated and crossed polarizers block light transmission. In Harsch minimum light transmission occurs in the deengergized (dark) state and maximum transmission occurs when the liquid crystal cells are energized (clear state). In Harsch three polarizers and two twisted nematic liquid crystal cells are arranged such that residual leakage of light through the upstream pair of polarizers and liquid crystal cell when minimum transmission is intended will be reduced by cooperation with the downstream liquid crystal cell and the further polarizer without substantially reducing transmission when in the clear (energized) state.
Alignment of three parallel directionally aligned polarizers and two twisted nematic liquid crystal cells (the five being arranged alternately in tandem to provide selective control of light transmission) also is disclosed in Fergason U.S. Pat. No. 3,918,796.
In such a twisted nematic liquid crystal cell, e.g., as is disclosed in U.S. Pat. Nos. Re. 29,684 and 4,039,254, nematic liquid crystal is located between a pair of generally fiat plates which are pretreated at a respective surface of each by parallel rubbing or by some other process to obtain generally parallel structural alignment (alignment of the directors) of the liquid crystal material relative to the rub direction. The plates are placed in parallel to each other such that the rub direction of one surface is perpendicular to the rub direction of the other, and the liquid crystal material between the plates tends to assume a helical twist. During use, the twisted nematic liquid crystal cell is placed between a pair of plane polarizers (also referred to as linear polarizers). Light incident on the first polarizer is linearly polarized thereby and directed through the twisted nematic liquid crystal cell to the second polarizer. In the absence of an electric field input to the twisted nematic liquid crystal cell, the plane of polarization is rotated, for example, ninety degrees as the light is transmitted through the cell. Such light transmission through a twisted nematic liquid crystal cell sometimes is referred to as wave guiding of the light. In the presence of an electric field sufficient to cause alignment of substantially all of the liquid crystal material in the cell with respect to such field, the plane polarized light incident on the cell is transmitted therethrough without such rotation. Depending on the orientation of the second polarizer (also sometimes referred to as the analyzer or analyzer polarizer) relative to the first polarizer, polarized light will be transmitted or blocked, as a function of alignment of the liquid crystal material in the cell and, thus, of whether or not electric field is applied. The light transmission and the control of light transmission usually is substantially the same for any wavelength of the light.
A surface mode liquid crystal cell, which is still another type of liquid crystal cell, and devices using such a cell are disclosed in U.S. Pat. Nos. 4,385,806, 4,436,376, Re. 32,521, and 4,540,243. In contrast to the wave guiding type operation of a twisted nematic liquid crystal cell, the surface mode liquid crystal cell operates on the principle of optical retardation, and, in particular, it operates to retard one of the two quadrature components (sometimes referred to as ordinary ray and extraordinary ray, respectively) of plane polarized light relative to the other. Thus, a surface mode liquid crystal cell effectively can rotate the plane of polarization of plane polarized light by an amount that is a function of a prescribed input, usually an electric field. A surface mode liquid crystal cell in effect is a variable optical retarder or variable wave plate that provides retardation as a function of the prescribed input. By locating the surface mode cell between plane polarizers, the transmission of light through a device including the surface mode cell can be controlled. An example of a surface mode cell used in an eye protection device is found in U.S. patent application Ser. No. 07/653,661. Another surface mode type liquid crystal cell is described in U.S. Pat. No. 4,582,396.
While these devices which employ polarizers to provide variable light transmission characteristics are improvements over devices offering fixed light transmission characteristics or mechanical shutter controlled variable light transmission, they suffer from an inability to control adequately the transmission of light obliquely incident on the welding lens, such as that emanating from a welding are adjacent the welder. Such adjacent welding are may be from welding equipment operated by another individual who is carrying out welding adjacent the location at which the first-mentioned welder is carrying out welding. While crossed or operatively crossed polarizers provide a high rate of extinction for light which is incident on the lens normal to the lens or at oblique angles lying substantially along the planes of polarization of the crossed polarizers, extinction at others regions may be substantially less, thus potentially causing annoyance or discomfort to the eyes of the welder from light sources falling in these regions.
As will be described further below, the present invention is directed to a variable optical transmission controlling device which offers optical extinction over a wide range of incident angles. The device is described in detail with respect to use in a welding helmet. However, it will be appreciated that the device may be employed in other environments and in other devices and systems for controlling transmission of electromagnetic energy broadly, and, in particular, optical transmission. As used herein with respect to the preferred embodiment, optical transmission means transmission of light, i.e., electromagnetic energy that is in the visible spectrum and which also may include ultraviolet and infrared ranges. The features, concepts, and principles of the invention also may be used in connection with electromagnetic energy in other spectral ranges.
The invention is especially useful for eye protection wherein protection is desired for light incident on the eye from any angle. Exemplary uses are in welding helmets, spectacles, goggles, and the like, as well as safety goggles for nuclear flash protection, for protection from hazards experienced by electric utility workers and for workers at furnace and electrical plant areas and at other places where bright light that could present a risk of injury may occur. The invention may be used in other devices and applications in which light attenuation is desired.
Shade number or shade is the characterization of darkness of a welding lens (sometimes referred to as a welding filter or simply a lens), for example, a larger shade number represents a darker, more light blocking (or absorbing) or less optically transmissive lens and a smaller shade number represents a less dark, less light blocking (or absorbing) or more optically transmissive lens. Generally optical transmission means transmission of light and the image or view carried by the light without substantial distortion of those images, e.g., due to scattering. Shade number is a term of art often used in the field of welding and especially welding lenses for eye protection.
Clear state or clear shade means the state of highest operating luminous transmittance (or light transmission) of the lens. This state corresponds to the state having the lowest shade number for the lens.
Dark state or dark shade is the lowest operating luminous transmittance (or light transmission) of the lens. This state corresponds to the state having the highest specified shade number for the lens. The invention is described below in some instances indicating that in the dark state no light is transmitted. While this may be desirable for some applications of the principles of the invention, it will be appreciated that for a welding lens in the dark state there will be some transmission so that the welder can see to do the welding while some light is blocked to provide the desired eye protection from damage, injury or the like by the light emitted during welding.
Intermediate state or intermediate shade is one that is not the clear state and the dark state. It may be between the clear state and the dark state, but this is not always necessary, for it may even be darker than the dark state if such operation could be achieved. According to the described embodiment of the invention, the intermediate state provides an intermediate level of light transmission/absorption, i.e., between the clear state and the dark state, and occurs during power failure, inadequate power, or power off status of the lens.
Power failure, failure, or fail state means absence of power being delivered to the lens. Power failure also may mean that inadequate power is being delivered to the lens to cause it to assume the desired state.
Off state is the condition of the lens when no electrical power is being supplied to the lens; the off state also is referred to as the power-off state. As is described below, the intermediate shade preferably occurs in the welding lens during the off state. The fail state of the lens also is the off state, i.e., no power is provided due to failure of the power supply; and the fail state also may occur when there simply is inadequate power available to drive or to energize the lens to the clear state and dark state.
Shutter response time is the time required for the circuitry associated with the lens to detect a sharp increase in incident light (e.g., due to striking of the welding arc, etc.) and to switch the lens from the clear state to the dark state.
Shutter recovery time is the time required for the circuitry associated with the lens to detect a sharp decrease in light (e.g., due to extinguishing of the welding arc, etc.) and to switch the lens from the dark state to the clear state.
Variable transmittance is the ability of the lens to be switched from one level of luminous transmittance (also referred to as transmission of light) to another level of luminous transmittance in response to a change in incident illumination.
Dynamic operational range or dynamic optical range is the operational range of the lens between the dark state and the clear state, e.g., the difference between the shade numbers of the dark state and the clear state.
The entire disclosures of all of the above-mentioned patents and patent applications are hereby incorporated by reference.